Random Access is an access process of a User Equipment (UE) before starting communication with a network. In the Long Term Evolution (LTE) system, the random access can be divided into two types: Synchronized Random Access and Non-synchronized Random Access. When the UE and the system achieve uplink synchronization, the random access process of the UE is referred to as synchronized random access; and when the UE and the system have not achieved uplink synchronization or lost uplink synchronization, the random access process of the UE is referred to as non-synchronized random access. Because when making non-synchronized random access, the UE has not obtained accurate uplink synchronization, one main characteristics of the difference between the non-synchronized random access and the synchronized random access is that the clock for the uplink transmission of the UE needs to be estimated and adjusted, to control the synchronization error to be within the length of the Cyclic Prefix (CP).
Under normal circumstances, the UE first performs downlink synchronization through a Synchronization Channel (SCH for short) after being started up to obtain a radio frame number, a location for receiving the sub-frame and a cell ID; and then detects a Broadcast Channel (BCH for short) to obtain the system information, which includes the configuration information of the Random Access Channel (RACH) channel, and finally performs uplink synchronization through the RACH channel to complete the process of accessing the system. This process belongs to non-synchronized random access.
In the 3rd Generation Partnership Project (3GPP) LTE protocol, a number of preamble sequences of the uplink random access are provided. In the uplink synchronization transmission process, the UE obtains the location of the RACH channel according to the radio frame and the location of the sub-frame determined in the downlink synchronization, and randomly selects one preamble sequence as a preamble for transmission from available preamble sequences; the base station side detects it to determine the timing advance for the uplink synchronization, and transmits it to the UE; the UE adjusts the timing for uplink transmission data according to the timing advance sent by the base station, to implement the time synchronization of the uplink channel.
The uplink random access preamble sequence specified in the LTE protocol is a Zadoff-Chu (ZC) sequence, and the uth root ZC sequence is defined as:
                    x        u            ⁡              (        n        )              =          ⅇ                        -          j                ⁢                              π            ⁢                                                  ⁢            u            ⁢                                                  ⁢                          n              ⁡                              (                                  n                  +                  1                                )                                                          N            ZC                                ,      0    ≤    n    ≤                  N        ZC            -      1      wherein, u is an index number of the root ZC sequence, NZC is the length of the ZC sequence and NZC is a prime number, the value thereof specified in the LTE protocol is 839 or 139.
As shown in FIG. 1, the random access sub-frame specified in the LTE protocol is comprised of three parts, which are the CP part, the RACH preamble sequence part and the Guard Interval (GT) part respectively. Wherein, the preamble sequence is derived by selecting different cyclic shifts (Ncs) based on the ZC sequence.
According to different cell coverage, the required CP lengths are different, and the lengths of the GTs and the preambles are also different. The existing LTE system supports five Formats, which are Formats 0-4 respectively, each Format corresponding to different cell coverage. The cell coverage radius is decided by both the cyclic shift of the sequence and the length of the GT.
In accordance with the protocol Format, the maximum cell radiuses supported by Format 0˜Format 4 calculated respectively are shown in Table 1. Wherein, TCP, TSEQ and TGT are the time-domain lengths of the CP part, the preamble sequence part and the GT part respectively; and Ts is a time unit specified by the LTE protocol, Ts=1/(15000×2048) second.
TABLE 1cell coverage radiuses supported by different formatscell radiusFormatTCPTSEQTGTsupportedFormat 0 3168 Ts   24576 Ts 2976 Ts14.5kmFormat 121024 Ts   24576 Ts15840 Ts77kmFormat 2 6240 Ts2 × 24576 Ts 6048 Ts30kmFormat 321024 Ts2 × 24576 Ts21984 Ts100kmFormat 4 448 Ts   4096 Ts 614 Ts3km
It can be seen from the table above, under the existing LTE protocol, the maximum cell radius which can be supported by the Physical Random Access Channel (PRACH) is 100 Km. However, as the LTE system is applied more and more widely, the power and the efficiency of the Radio frequency device increase gradually, and it needs to support a larger cell radius. For such requirement, the LTE protocol can not satisfy that at present.
In order to increase the cell radius, a solution which is available for consideration includes: modifying the Format of the random access sub-frame of the protocol, increasing the time-domain lengths of the CP part, the preamble sequence part and the GT part etc. However, the change of the random access sub-frame will certainly result in the PRACH channel occupying more physical resources, which reduces the performance of the service channel of the system; at the same time, the Time Division Duplexing (TDD) LTE can not support the time-domain length of a larger random access sub-frame under the existing frame format, i.e., such a solution can not be applied in the TDD LTE system.